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Figure 1. Lethiscus stocki skull, drawing from Pardo et al. 2017 and colorized here. Note the loss of the postfrontal and the large orbit. Pardo et al. nest this taxon between Acanthostega and Pederpes in figure 3. There is very little that is plesiomorphic about this long-bodied legless or virtually legless taxon. Thus it should nest as a derived taxon, not a basal plesiomorphic one.

Lethiscus is indeed very old (Middle Viséan)but several reptiles are almost as old and Tulerpeton, a basal amniote, comes from the even older Late Devonian. So the radiation of small burrowing and walking tetrapods from shallow water waders must have occurred even earlier and Tulerpeton is actually the oldest crown tetrapod.

Figure 3. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles), resurrecting an old idea not supported in the LRT. Actually not much of this topology is supported by the LRT.

Pardo et al. nested Lethicus
between Acanthostega (Fig. 4) and Pederpes (Fig. 3) using a matrix that was heavily weighted toward brain case traits. Ophiderpeton and Oestocephalus (Fig. 2) were not included in their taxon list, though the clade is mentioned in the text: “Overall, the skull morphology demonstrates underlying similarities with the morphologies of both phlegethontiid and oestocephalid aïstopods of the Carboniferous and Permian periods.” So I’m concerned here about taxon exclusion. No other basal tetrapods share a lateral temporal fenestra or share more cranial traits than do Lethiscus, Ophiderpeton, Oestocephalus and Rileymillerus. All bones are identified here as they are in Pardo et al. so bone ID is not at issue. I can’t comment on the Pardo team’s braincase traits because so few are examined in the LRT. Dr. Pardo said they chose taxa in which the brain case traits were well known and excluded others.

Figure 4. Acanthostega is basal to Lethiscus in the Partdo et al. tree.

Pardo et al. consideredthe barely perceptible notch between the tabular and squamosal in Lethiscus (Fig. 1) to be a “spiracular notch” despite its tiny size. I think they were reaching beyond reason in that regard. They also note: “The supratemporal bone is an elongate structure that forms most of the dorsal margin of the temporal fenestra, and is prevented from contacting the posterior process of the postorbital bone by a lateral flange of the parietal bone.” The only other taxon in the LRT that shares this morphology is Oestocephalus, Together they nest within the Lepospondyli (Fig. 3) in the LRT. I think it is inexcusable that Pardo et al. excluded Ophiderpeton and Oestocephalus.

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Summarizing,
Pardo et al. report, “The braincase and its dermal investing bones [of Lethiscus] are strongly indicative of a very basal position among stem tetrapods.” and “The aïstopod braincase was organized in a manner distinct from those of other lepospondyls but consistent with that seen in Devonian stem tetrapods.” It should also be noted that the skull, body and limbs were likewise distinct from those of other lepospondyls, yet they still nest with them in the LRT because no other included taxa (1018) share more traits. ‘Distinct’ doesn’t really cut it, in scientific terms. As I mentioned in an email to Dr. Pardo, it would have been valuable to show whatever bone in Lethiscus compared to its counterpart in Acanthostega and Oestocephalus if they really wanted to drive home a point. As it is, we casual to semi-professional readers are left guessing.

Pardo et al. references the clade Recumbirostra.Wikipedia lists a number of microsaurs in this clade with Microbrachis at its base, all within the order Microsauria within the subclass Leposondyli. Pardo et al. report, “Recumbirostrans and lysorophians are found to be amniotes, sister taxa to captorhinids and diapsids.” The LRT does not support this nesting. Pardo et al. also report, “This result is consistent with early understandings of microsaur relationships and also reflects historical difficulties in differentiating between recumbirostrans and early eureptiles.”Yes, but the later studies do not support that relationship. Those early understandings were shown to be misunderstandings that have been invalidated in the LRT and elsewhere, but now resurrected by Pardo et al.

Figure 5. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

A side note:The recent addition of several basal tetrapod taxa has shifted the two Phlegethontia taxa (Fig.5) away from Colosteus to nest with Lethiscus and Oestocephalus, their traditional aistopod relatives. That also removes an odd-bedfellow, tiny, slender taxon from a list of large robust stem tetrapods.

Figure 1. Apateon overall and the skull in palatal and dorsal views. This taxon nests between Doleserpeton and Gerobatrachus in the LRT.

Apateon pedestris(von Meyer 1844, Early Permian, 295mya; 12 cm in length) was long considered a temnospondyl in the family Branchiosauridae. HereApateon nests between Doleserpeton and Gerobatrachus in the lepospondyl lineage of frogs, like Rana and salamanders like Andrias.

Resembling a small salamander with a long, laterally flattened tail, Apateon had a shorter rostrum and large orbits than Doleserpeton. The pineal opening was larger. The ilium was more erect. The pubis was missing. The ectopterygoid did not contact the maxilla and the palatine did so only with a narrow process. At present, no other taxa in the LRT (978 taxa) do this.

Small scales covered the body. Three pairs of external gills were present for underwater respiration. Many species are known, as well as a good ontogenetic series.

Anderson 2008 reported, “Branchiosaurs [including Apateon]are closely related to amphibamids, if not included in the latter group, and have been suggested to be closely related to salamanders because of shared similarities in the sequence of cranial ossification.”

“New transitional fossils like the stem batrachian Gerobatrachus have filled in the morphological gap between amphibamid temnospondyls and the earliest frogs and salamanders, and this portion of the lissamphibian origins question appears very well supported.”

The LRT recoversAmphibamus much closer to the base of the lepospondyls, about 5 nodes distant from Apateon. Of course, neither are closely associated with temnospondyls in the LRT, despite the open palate, otic notch and other convergent traits.

NeotonyThe apparent lack of gill-less adults among all of the apparent larval gilled specimens of Apateon was a cause of consternation for awhile. The new largest specimen (Frobisch and Schoch 2009) appears to indicate an adult specimen. It had partially interdigitating and tight sutures of the skull roof, a high degree of ossification and differentiation of the postcranium as compared to smaller larval specimens. Uncinate processes indicate that this specimen represents an adult. However, it lacks ossifications of the exoccipitals and quadrates, intercentra, and the coracoid as seen in metamorphosed specimens. Frobisch and Schoch conclude, “The anatomical evidence at hand clearly indicates that both life history strategies, metamorphosis and neoteny, were established in Paleozoic branchiosaurids.”

ReferencesAnderson JS 2008. Focal Reviews: The Origin(s) of Modern Amphibians. Eovlutionary Biology 35:231-247.
Anderson JS et al. 2008. A stem batrachian from the Early Permian of Texas
and the origin of frogs and salamanders. Nature 453 (7194): 515–518.Frobisch N and Schoch RR 2009. The largest specimen of Apateon and the life history pathway of neotony in the Paleozoic temnospondyl family Branchiosauridae. Fossil Record 12(1):83-90.von Meyer H 1844. Briefliche Mittheilung an Prof. Bronn gerichtet. Neues Jahrbuch für Geognosie, Geologie und Petrefakten-Kunde 1844: 329-340.

Updated June 13, 2017 with the fact that Diplovertebron is the same specimen I earlier illustrated as Gephyrostegus watsoni. And the Watson 1926 version of Diplovertebron (Fig. 1) was so inaccurately drawn (by freehand) that the data nested is apart from the DGS tracing. Hence this post had deadly errors now deleted.

Figure 2. The gradual loss of basal tetrapod fingers. Unfortunately fingers are not known for every included taxon. Odd Tulerpeton with 6 fingers may result from taphonomic layering of the other manus peeking out below the top one. See figure 6. Mentally delete Diplovertebron from this chart.

The presence of five manual digits
in Balanerpeton(Figs. 4, 5) sheds light on their retention in Acheloma + Cacops. There is a direct phylogenetic path between them (Fig. 2). Note that all other related clades lose a finger or more. Basal and stem reptiles also retain five fingers.

As reportedearlier, finger five was lost in amphibians,while finger one was lost in temonospondyls. Now, based on the longest metacarpal in Caerorhachis and Amphibamus (second from medial), apparently manual digit one was lost in that clade also, distinct from the separate frog and microsaur clades. In summary, loss from five digits down to four was several times convergent in basal tetrapods.

Figure 5. DGS recovers five fingers in Balanerpeton with a Diplovertebron-like phalangeal pattern. Two 5-second frames are shown here.

Finally, we have to talk aboutTulerpeton (Fig. 6). The evidence shows that the sixth manual digit is either a new structure – OR – all post-Devonian taxa lose the sixth digit by convergence, since they all had five fingers. Finger 6 has distinct phalangeal proportions, so it is NOT an exposed finger coincident rom the other otherwise unexposed hand in the fossil matrix.

Figure 6. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here. Digit 6 is either a new structure, or a vestige that disappears in all post-Devonian taxa.

Platyhystrix was a dissorophid lepospondyl anamniote that had a dorsal sail (Figs. 1, 2 ), not quite like those of its more famous reptilian/synapsid contemporaries, Dimetrodon and Edaphosaurus.

There must have been something in the air back then,
and those dorsal sails were there to catch it!

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts. And the Lewis and Vaughn 1965 dorsal sail, distinct from the others in figure 2. The skull here appears to have a confluent naris and antorbital finestra, as in Acheloma, but there are other bones missing there, too, like most of the maxilla.

Dissorophids are traditionally nested withtemnospondyls, but here, at the large reptile tree (LRT, now 959 taxa), they arise from a sister to the basal seymouriamorph, Utegeniaand continue to be generally smaller taxa (< 60cm).

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines. That old skull from Williston 1911 is missing the central area, here imagined from the more complete specimen in figure 1.

Distinct from Achelomathe skull of Platyhysterix does not appear to have giant palatal fangs, or such large marginal teeth. The jugal nearly separates the postorbital from the supratemporal. The postorbital is larger and much knobbier.

Like Acheloma
The rostrum may include a confluent nairs/antorbital fenestra, a constricted rostrum (in dorsal view), a naris of similar laterally wavy shape, robust premaxillary ascending processes, large tabulars and other traits relatively exclusive to these two.

A fair amount of reassembly
is required of the Platyhystrix skull. The random neural spine below the lower right jaw line allies the skull with specimens that also have long neural spines.

Wouldn’t it be interesting
to see hatchlings and juveniles of Platyhystrix? It is widely considered, along with its double-armored kin, Dissorophus, to have been fully terrestrial. So, did these two have a swimming tadpole stage? And then develop spines and armor in adulthood? Or did they converge with reptiles, laying protected eggs on land, skipping the tadpole stage? Let’s keep an eye out for little finbacks.

ReferencesBerman DS, Reisz RR and Fracasso MA 1981. Skull of the Lower Permian dissorophid amphibian Platyhystrix rugosus. Annals of the Carnegie Museum 50 (17):391-416.Case EC 1911. Revision of the Amphibia and Pisces of the Permian of North America. Publ. Carnegie Inst. Washington 146:1-179.Dilkes DW and Reisz R 1987.Trematops milleri identified as a junior synonym ofAcheloma cumminsi with a revision of the genus. American Museum Novitates 2902.Lewis GE and Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville area, Colorado, with a section on Footprints from the Cutler Formation by Donald Baird: U.S. Geol. Survey Prof. Paper 503-C, p. 1-50.Williston SW 1911a. A new family of reptiles from the Permian of New Mexico. American Journal of Science 31:378-398.Williston SW 1911b. American Permian vertebrates. University of Chicago Press: 145 pp.

This blogpost is somewhat outside of the reptile arena,
but marginally pertinent, as you’ll see, nevertheless.

A recent paper by Sanchez et al. 2016
reports that none of the known Acanthostega specimens (Fig. 1) are fully mature, even at six years old. As we learned earlier, these Latest Devonian lobe fins and basal tetrapods were not involved in the transition to land, which occurred in the early Middle Devonian, as documented by footprints. But, as late survivors of that transitional phase, they do give us a good view as to what happened tens of millions of years earlier.

Figure 1. Size comparisons of lobe-find fish, their eggs and embryos. Latimeria at top, Ichthyostega in the middle, Acanthostega at bottom with hypothetical adult with reduced tail fin, egg and hatchling also shown.

The Sanchez abstract reports“A long early juvenile stage with unossified limb bones, during which [Acanthostega] individuals grew to almost final size, was followed by a slow-growing late juvenile stage with ossified limbs that lasted for at least six years in some individuals. The late onset of limb ossification suggests that the juveniles were exclusively aquatic,”

Hypothetical hatchlingsbased on hip dimensions and analogy with Latimeria (Fig. 1) suggest adults were 5-6x longer than juveniles. In Latimeria the ratio is 9x. Coelocanths can live for 48 years. Gestation can be 12-13 months. No information exists for the age of sexual maturity, but it could be as late as 20 years in Latimeria.

So the Sanchez et al. discoveriesseem to make sense in terms of phylogenetic bracketing. Lobefins take a long time to mature and they live a long time thereafter.

Brachydectes newberryi(Cope 1868, AMNH 6941; latest Carboniferous; 300 mya; Fig. 1-4) was long considered a lysorophian amphibian with a tiny skull, an extremely long snake-like torso, vestigial limbs and a very short tail. You find them in eastern Kansas.

Figure 1. Brachydectes overall and skull in four views.

A recent PlosOne article
by Pardo and Anderson (2016) studied the skull of Brachydectes (Fig. 3) using micro CT scanning. They report, “Contra the proposals of some workers, we find no evidence of expected lissamphibian synapomorphies in the skull morphology in Brachydectes newberryi, and instead recognize a number of derived amniote characteristics within the braincase and suspensorium. Morphology previously considered indicative of taxonomic diversity within Lysorophia may reflect ontogenetic rather than taxonomic variation.” Later they wrote, “an expansive phylogenetic analysis is outside the scope of this study and will appear elsewhere.”

Earlierin the large reptile tree (LRT), Brachydectes nested between Adelospondylus and Eocaecilia, which also has a long snake-like torso, but composed of far fewer and individually much longer vertebrae and a distinct skull architecture. A large, but not exhaustive, selection of basal amniotes was tested and none attracted Brachydectes as much as the two lissamphibians listed above, given the prior data of a line drawing of the skull (Fig. 2) by Marjanovic and Laurin 2013 derived from Wellstead C F 1991.

Figure 2. Brachydectes skull data from a line drawing produced by Marjanović and Laurin 2013. Most leposponysls have a very narrow parasphenoid process and large interptyergoid vacuities, but eocacaecilians expanded this bone and reduced the vacuities like Brachydectes did.

Figure 1. Brachydectes newberryi has some difficult to identify bones just aft of the orbit due to fusion and reduction. Brachydectes (Laysorophus) elongatus (Fig. 2) provides Rosetta Stone clues as to what is happening in this clade.

The new data
(Figs 2,3 ) are not too far off from the Wellstead C F 1991 data. Notably the tabular no longer extends ventrally alongside the squamosal as it does in the larger specimen. Does this represent a break? or fusion? Or phylogenetic difference? Below (Fig. 3) is the new data on KUVP 49541, plus a reinterpretation of skull sutures based on the micro CT scans. The nesting of the new Brachydactes does not shift in the LRT. It is still a lissamphibian close to microsaurs and caecilians. That’s a broad range, indicative of a long list of yet to be found taxa.

Pardo and Anderson’s reconstruction
(Fig. 3) does not include the coronoid or lateral exposure of the splenial. Pardo and Anderson note the single supraoccipital compares well with that of various basal reptiles, and indeed it does. The occipital arch of other lissamphibians consists of only paired exoccipitals,.. until you include microsaurs.

More on supraoccipital homologies
According to Pardo and Anderson, “the presence of a well-developed median supraoccipital is restricted to the amniote crown and recumbirostran ‘microsaurs’. Although the supraoccipital of Brachydectes and ‘microsaurs’ has traditionally been considered convergent with the amniote supraoccipital, new data from μCT have demonstrated that the ‘microsaur’ supraoccipital shares a number of morphological details with early amniotes, and early eureptiles in particular, and is likely homologous with the amniote element. This homology does not extend far down the amniote stem, as seymouriamorphs lack a supraoccipital and ‘anthracosaurs’ generally exhibit paired elements within the synoptic tectum.”

Noteworthy:In the LRT, microsaurs are sisters to the clade that includes Adeospondylus, Brachydectes and Eocaeceila. That’s a great deal of phylogenetic distance, but not as great as any other pairing in the LRT. Perhaps more taxa will fill the apparent gaps someday.

Figure 4. Four sizes of Brachydectes in situ. Here, unfortunately, the authors have penned in the sutures, negating any possibility of any reviewer to judge whether they were drawn correctly or not.

Pardo and Anderson also report“neurocranial morphology does not support a close relationship between Brachydectes and lissamphibians.” Admittedly, Brachydectes is indeed quite different from its sisters…yet it is not closer to other tested taxa in the LRT. If you look at various microsaurs and other lissamphibians, you get a wide range of morphologies at every node.

By noting various key features in contention with the traditional relationship. Pardo and Anderson essentially ‘put the cart before the horse.’ They waited to do the phylogenetic analysis, when they should have done that analysis before publishing. Homoplasy is rampant in tetrapods. I think they fell prey to yet another example. Only analysis, at present, settles all issues.

Pardo and Anderson then report, “Morphology of the braincase of Brachydectes suggests a close relationship with the brachystelechid ‘microsaurs’Carrolla craddockiandQuasicaecilia texana, within theRecumbirostra.” These two are new to me and untested in the LRT. Wikipedia nests them with Batropetes, which has long legs, and a horned-lizard type body, only distantly related to Brachydectes in the LRT. The skull of Quasicaecilia is shown here, but no post-crania is shown. Recumbirostran microsaurs, are considered the earliest known example of adaptation to head-first burrowing in the tetrapod fossil record. I wish the sister candidates offered by Pardo and Anderson were long and snake-like, but they are not. Deletion of post-cranial traits from the LRT does not shift the placement of Brachydectes within the LRT.

Figure 5. Original interpretation of Brachydectes, KUVP 49541, by Pardo and Anderson. Colors added for clarity and to match micro CT scan.

Earlier we looked at the evolution of the frog, Rana. And it continues to be the most popular blog post of the past year.

Today, after adding Rana to the matrix of the large reptile tree (still not updated), I think it’s time we looked at the antorbital fenestra of Rana, and the lateral temporal fenestra as well (Fig. 1).

Figure 1. Rana, the bull frog, with naris in red, orbit in purple, antorbital fenestra in dark blue and lateral temporal fenestra in orange. The reduction of the the skull bones in Rana created these fenestrae.

One usually thinks of additional skull fenestrae in the province of reptiles. As we saw earlier, the antorbital fenestra comes and goes in several reptiles. So does the lateral temporal fenestra. Amphibians (non-amniote tetrapods) typically do not have skull fenestrae. Neither to most basal reptiles.

Relative to the body, the skull of Rana is enormous. So are the hind limbs. Frogs leap, as everyone knows, and if the skull is going to be large it also has to be lightweight to enable longer leaps. So the skull bones are reduced to their bare minimum creating fenestrae.